Methanol is one of the major chemical raw materials, ranking third in volume behind ammonia and ethylene. Worldwide demand for methanol as a chemical raw material continues to rise especially in view of its increasingly important role (along with dimethyl ether) as a source of olefins such as ethylene and propylene and as an alternative energy source, for example, as a motor fuel additive or in the conversion of methanol to gasoline.
Another commercially important source of hydrocarbons for use as fuels, lubricants and other petrochemical feedstocks are liquids derived via the Fischer-Tropsch procedure. The Fischer-Tropsch process was developed in the 1920's as a way of synthetically producing higher hydrocarbons from lower hydrocarbon-containing feedstocks. Initially the process was centered on producing gasoline range hydrocarbons as automotive fuels. More recently, the Fischer-Tropsch process has been viewed as a viable method for preparing even heavier hydrocarbons such as diesel fuels, and more preferably waxy molecules for conversion to clean, efficient lubricants.
Both methanol (as well as dimethyl ether) and Fischer-Tropsch liquids can be produced via the catalytic conversion of a gaseous feedstock comprising hydrogen, carbon monoxide and carbon dioxide. Such a gaseous mixture is commonly referred to as synthesis gas or “syngas”.
Methanol is typically produced from the catalytic reaction of syngas in a methanol synthesis reactor in the presence of a heterogeneous catalyst. For example in one synthesis process, methanol is produced using a copper/zinc catalyst in a water-cooled tubular methanol reactor. In methanol production, syngas undergoes three reactions, only two of which are independent. These reactions are:2H2+COCH3OH  (1)3H2+CO2CH3OH+H2O  (2)H2O+COH2+CO2  (3)
As can be seen from Reactions #2 and #3, CO2 can participate in methanol synthesis. Nevertheless, it is desirable to minimize the amount of CO2 in the syngas for several reasons. In the first place, a low CO2 content in the syngas results in a more reactive mixture for methanol synthesis provided the CO2 content is at least about 2%. Furthermore, less CO2 results in lower consumption of hydrogen and lower production of water. Lower water production is useful in applications where some relative small amounts of water can be present in the methanol product such as, for example, in connection with a methanol to olefins (MTO) process. Production of methanol with low water content thus eliminates the need to distill water from the syngas product methanol.
The syngas stoichiometry for methanol synthesis from syngas is generally described by the following relationship known as the “Stoichiometric Number” or SN.SN=(H2−CO2)/(CO+CO2)
The value of SN theoretically required for methanol synthesis is 2.0. However, for commercial production of methanol from syngas, it is desirable that the value for SN range from about 2.05 to 2.10. Dimethyl ether (DME) may also be produced from syngas using chemistry similar to that used for methanol synthesis.
Syngas may also be used as the starting feedstock for production of Fischer-Tropsch liquids. The Fischer-Tropsch process is a well-known process, and the reaction conditions for it have been described in the literature. For example, temperatures may range from about 175° C. to 400° C., preferably from about 180° C. to 250° C. Pressures from about 1 to 100 bar, more preferably from about 15 to 40 bar are used. The catalysts employed in the Fischer-Tropsch process are iron and cobalt. Promoters like rhenium, zirconium, manganese and the like are commonly used to improve various aspects of catalytic performance. These catalysts and promoters are typically supported on a particulate material such as alumina or titania.
Starting with syngas, the Fischer-Tropsch synthesis proceeds according to the following reaction:2H2+CO(1/n)(CnH2n)+H2OAs can be seen from this reaction, CO2 does not participate in the synthesis of Fischer-Tropsch liquids and can generally be viewed as an inert ingredient in the F-T synthesis procedure. The H2/CO ratio desired for synthesis of Fischer-Tropsch liquids is about 2.0.
Synthesis gas useful for either methanol/dimethyl ether synthesis or production of Fischer-Tropsch liquids can be obtained from a source of lower hydrocarbons, such as natural gas, via a procedure known as reforming. Various commercial reforming technologies are known in the art. All involve reaction of hydrocarbons such as methane with steam, oxygen or both
Steam reforming (SR) involves the reaction of a hydrocarbon feedstock, e.g., natural gas, with steam over a suitable reforming catalyst. In general, this reaction proceeds according to the following general reaction scheme:CnHm+nH2OnCO+[n+m/2]H2 with the water gas shift reaction (Reaction #3 above) also taking place to produce CO2. For methane, this principal reaction becomes:CH4+H2OCO+3H2.
All of the heat needed to drive this endothermic reforming reaction is supplied externally to the catalyst bed, and no oxygen is used in the feedstock or introduced into the process. Commercial SR technologies utilize a nickel catalyst and a relatively large ratio of steam to carbon in order to prevent coking of the catalyst. The steam reforming operation generally produces syngas with a Stoichiometric Ratio SN which is above that best suited for methanol synthesis and also has excessive hydrogen beyond that which is needed for either methanol or Fischer-Tropsch synthesis. Furthermore, the CO2 content in the resulting syngas can be excessive for production of either methanol for which low water content may be desired or for F-T synthesis.
Autothermal reforming (ATR) involves the addition of air or oxygen with relatively smaller proportions of steam to a hydrocarbon feedstock. Reaction of hydrocarbon with oxygen proceeds according to the following general reaction schemes:CnHm+(n/2)O2nCO+(m/2)H2 andCnHm+(n+m/4)O2nCO2+m/2H2O
When methane is the hydrocarbon undergoing oxidative reforming, these reactions become:CH4+½O2CO+2H2 andCH4+2O2CO2+2H2O
Autothermal reforming employs both steam reforming and oxidative reforming of the hydrocarbon feed. The exothermic oxidation of the feedstock hydrocarbons generates sufficient heat to drive the endothermic steam reforming reaction over the catalyst bed. The ATR procedure is thus run at relatively high temperatures and pressures with a relatively low steam to carbon ratio. The syngas resulting from autothermal reforming generally has a slight surplus of hydrogen for F-T synthesis but insufficient hydrogen and hence a Stoichiometric Ratio SN which is not high enough for methanol synthesis. The CO2 content of the syngas from ATR processes, however, is fairly low, as is desirable for both methanol and F-T synthesis.
It is also known to run a combination of separate steam reforming and autothermal reforming operations for syngas production in order to provide syngas with a more suitable Stoichiometric Ratio SN for methanol production. In such a combined reforming (CR) procedure, steam is mixed with the hydrocarbon feed which is partially converted to syngas in a steam reformer with externally supplied heat. Autothermal reforming is then used to convert the rest of the hydrocarbon feedstock, either in series and/or in parallel with the SR operation. Heat from the ATR stage effluent can be used to supply the external heat needed for the SR stage. The Stoichiometric Ratio SN for the resulting syngas from CR can be made ideal for methanol synthesis, but the syngas has an excessive amount of hydrogen for Fischer-Tropsch synthesis. The relatively high steam to carbon ratio used for the SR portion of the combined reforming operation also results in a relatively high CO2 content in the syngas produced due to the water gas shift reaction. Such CO2 content is higher than is desirable for low-water methanol synthesis wherein minimization of water production is needed.
Another known reforming process involves primarily partial oxidation of a hydrocarbon feed with an oxygen-containing gas. Although catalytic partial oxidation reforming procedures are known, for purposes of this invention, partial oxidation reforming takes place in the absence of a catalyst. Due to the absence of a catalyst, partial oxidation (POX) reforming can operate at very high temperatures with little or no steam addition to the feedstock. Higher pressures than are used in ATR operations can be employed in POX reforming. However, the syngas composition resulting from POX reforming is generally deficient in hydrogen for either methanol or F-T synthesis, resulting in SN and H2:CO numbers below 2. On the other hand, the CO2 content of the resulting syngas is generally very low which is ideal for F-T synthesis but below the optimum value for methanol synthesis.
The three reforming operations hereinbefore described which employ an oxygen-containing gas to generate heat as a consequence of the reforming reaction are known as oxygen-blown reforming (OBR) processes. These OBR processes generally produce syngas at a higher temperature and pressure than the syngas produced by SR reforming processes.
It is known in the prior art to utilize various combinations of reforming operation types and procedures in the preparation of syngas mixtures which can be converted, for instance, into oxygenates. For example, Texaco's U.S. Pat. No. 5,496,859 discloses a method for the production of a “stoichiometric ratioed syngas”. The method comprises partially oxidizing a gaseous feedstock containing substantial amounts of methane in a gasifier to produce a hot synthesis gas stream that is passed in indirect heat exchange through a steam reforming catalytic reactor. A portion of the steam reforming reaction products are mixed with the cooled gasifier synthesis gas stream exiting the steam reforming catalytic reactor to form a stoichiometric ratioed synthesis gas. The stoichiometric ratioed synthesis gas stream can then be passed into a methanol synthesis unit at substantially the specifications for optimal methanol production with little or no external compression. The stoichiometric ratio, SN, in the syngas produced is said to range from 1.9 to 2.1. Syngas having an SN of 1.9 and an H2/CO ratio of 2.52 are exemplified in the '859 patent. The exemplified syngas has a excessively high water content of 22%, or on a water-free basis a relatively high CO2 content of 5.3%.
Haldor Topsoe's U.S. Pat. No. 6,224,789 and related publication [Aasberg-Petersen et al; Applied Catalysis, A: General (2001), 221 (1-2), pp 379-387] both disclose an arrangement similar to that of the Texaco '859 patent wherein effluent gas from an ATR unit circulates around and supplies heat to the HTCR (a heat exchanger version of an SR reactor), but does not undergo chemical reaction there. The exemplified Haldor Topsoe process provides a syngas with an SN stoichiometric ratio of 3.66 and an H2/CO ratio of 3.25.
Shell's U.S. Published Application No. 2004/241,086 discloses a process for the preparation of syngas from a gaseous hydrocarbon feedstock by (a) partial oxidation of a part of the feedstock and (b) steam reforming of another part of the feedstock. The mixture obtained in step (b) may be directly combined with the product gas as obtained in step (a). No description of feedstock or syngas composition characterized by component concentration is given.
ICI's U.S. Pat. No. 5,252,609 discloses a syngas production process involving the SR and OBR treatment of two separate hydrocarbon feedstock streams. Such a process comprises (a) steam reforming a first stream of desulfurized hydrocarbon feedstock, optionally followed by secondary reforming using an oxygen-containing gas and then cooling, (b) steam reforming a second stream of the feedstock, preferably adding a hydrogen-containing gas, and then subjecting the product to partial oxidation with an oxygen-containing gas and then cooling, and (c) mixing the two cooled product streams.
Shell's WO 04/092062 and WO 04/092063 both disclose other syngas production processes involving a combination of different types of reforming operations. Such processes comprise (a) partial oxidation of a carbonaceous feedstock thereby obtaining a first gaseous mixture of hydrogen and carbon monoxide, (b) steam reforming a carbonaceous feedstock, wherein the steam to carbon molar ratio is below 1, to obtain as a steam reforming product, (c) feeding the steam reforming product to the upper end of the partial oxidation reactor to obtain a mixture of the effluent of step (a) and the steam reformer product, and (d) providing the required heat for the steam reforming reaction in step (b) by convective heat exchange between the mixture obtained in step (c) and the steam reformer reactor tubes, thereby obtaining a hydrogen and carbon monoxide containing gas having a reduced temperature.
Praxair/Standard Oil's U.S. Pat. No. 6,402,988 discloses the following: “An exothermic reaction and an endothermic reaction are thermally combined in a reactor having at least one oxygen selective ion transport membrane that provides the exothermic reaction with oxygen from an oxygen-containing gas such as air. The thermal requirements of the endothermic reaction are satisfied by the exothermic reaction. Dependent on the reactor design employed, the exothermic and endothermic reactions may be gaseously combined”. The exothermic reaction is partial oxidation; the endothermic reaction is steam reforming.
Davy McKee's WO 93/15999 shows an arrangement, with steam reforming (SR) and partial oxidation (POX) units in parallel. The effluents are combined and in this arrangement are sent to a secondary reforming zone for further reduction in product methane content.
Notwithstanding these various reforming arrangements of the prior art, it would still be desirable to identify additional syngas preparation techniques which involve combinations of the various types of natural gas reforming operations in order to realize syngas blends suitable for both oxygenate preparation and Fischer-Tropsch synthesis. In particular, it would be desirable to operate a steam reformer in a way which takes advantage of its strengths and to also operate an oxygen-blown reforming unit under conditions which take advantage of its strengths. It would then be advantageous to combine the syngas effluent from such reforming operations in a way and under conditions which result in a syngas product blend having stoichiometry of components that is optimized for subsequent production of either methanol/dimethyl ether oxygenates or Fischer-Tropsch liquids.